Generally, compressing an audio file, in particular a music file, using a lossy encoding scheme, such as MP3, leads to a decreased audio quality. The audio cannot be restored in its original quality, because due to the compression, for instance, high frequencies are lost, transients are distorted or weakened and other artifacts are introduced.
Early attempts to address this problem suggest recovering the original audio quality by using bandwidth extension techniques, as disclosed, for instance, in U.S. Pat. No. 7,916,876 and EP1915026. More recent attempts to address this problem suggest the use of equalization or bass maximization techniques, as disclosed, for instance, in U.S. Pat. Nos. 6,865,430 and 6,606,388. U.S. Pat. No. 9,135,920 discloses a mid/side decomposition of a stereo signal, which is a very simple improvement compared to modifying the whole signal.
The conventional methods for improving the quality of compressed audio files, in particular music files, rely on bandwidth extension only to regenerate high frequency and/or general sound effects to increase perceptual quality (e.g., equalization). Thus, conventional bandwidth extension systems are mostly implemented consisting of a separate step to detect the cutoff frequency and, subsequently, to estimate the parts of the audio file above this estimated cutoff frequency lost due to compression. However, high frequency audio content in general is perceptually of little relevance (at frequencies above 12 kHz many people cannot perceive sound at all) and the detection of the actual high-frequency cut-off frequency is a difficult task. Moreover, other important effects on the perceived quality of the music that are not addressed by the conventional methods are the so-called “holes” in the spectrogram which create audible artifacts (“musical noise”, “birdie sounds”) and the loss of the presence of attacks.
Thus, there is a need for an improved audio signal processing apparatus and method which allow, in particular, enhancing the quality of compressed audio signals.